Nitric oxide (NO) is synthesized enzymatically from arginine in numerous tissues and cell types by a family of enzymes, collectively known as nitric oxide synthase (NOS, E.C. 1.14.13.39). Three principal isoforms of this enzyme have been isolated and characterized, each associated with different physiological functions: the immune response (inducible NOS or iNOS), smooth muscle relaxation (endothelial NOS or eNOS), and neuronal signaling (neuronal NOS or nNOS). All of these isoforms utilize NADPH, FAD, FMN, (6R)-5,6,7,8-tetrahydrobiopterin and heme as cofactors.
Overproduction of NO has been a factor in numerous disease states. NO overproduction by nNOS has been implicated in strokes, migraine headaches, Parkinson's disease, Alzheimer's disease, and with tolerance to and dependence on morphine. iNOS-mediated overproduction of NO has been associated with development of colitis, tissue damage and inflammation, and rheumatoid arthritis.
Animal studies and early clinical trials suggest that NOS inhibitors could be therapeutic in many of these disorders; however, because of the importance of nitric oxide to physiological functioning, potent as well as isoform-selective inhibitors are essential. nNOS inhibition has been targeted for treatment of strokes and Parkinson's disease, and iNOS inhibition for the treatment of septic shock and arthritis. Although there may be pathologies associated with overactivity of eNOS, blood pressure homeostasis is so critical that most investigators believe that therapeutically useful NOS inhibitors should not inhibit eNOS.
Excellent inhibitory potency and selectivity for nNOS over eNOS and iNOS have been achieved with certain prior art nitroarginine dipeptide amides that have an amine-containing side chain (cpds. 1-3 in the cited reference). See Huang, H.; Martasek, P.; Roman, L. J.; Masters, B. S. S.; Silverman, R. B. Nω-Nitroarginine-Containing Dipeptide Amides. Potent and Highly Selective Inhibitors of Neuronal Nitric Oxide Synthase. J. Med. Chem. 1999, 42, 3147-53.
The most potent nNOS inhibitor among these compounds is L-ArgNO2-L-Dbu-NH2 (1) (Ki=130 nM), which also shows excellent selectivity over eNOS (>1500-fold) and 192-fold selectivity over iNOS. Further peptidomimetic modifications are, however, invariably necessary before such compounds can be therapeutically useful. Generally, peptides have poor bioavailability and, for that reason, are often unsuccessful as drug candidates.